The present invention relates to optoacoustic spectroscopy and more particularly to optoacoustic spectroscopy for detecting trace gases at the part-per-billion level.
There is interest in optoacoustic spectroscopy of detecting trace gases at the part-per-billion level. However, there is a problem of large window background signals in using conventional optoacoustic cells. Many attempts have been made to eliminate this background signal. These attempts include the differential cell, multipass cell, and windowless resonant cell.
The window background signal in optoacoustic spectroscopy arises whenever a modulated light source passes through any window material. In conventional optoacoustic cells, window background is generated from the two windows at each end of the cell. This signal is thought to be generated at the window-gas interfacial region, with the magnitude of this signal dependent on the heat capacity of the window material. Given the choice of the window materials used in the IR region (sapphire, germanium, ZnSe, NaCl, KCl, etc.), all have similar heat capacities making the window background problem ubiquitous.
The obvious solution to the window background problem would be to eliminate the windows in a non-resonant cell altogether. However, in conventional non-resonant cells this is not possible due to the need to confine the pressure wave in the cell such that this energy can be used to move the diaphragm of the microphone. Except for an infinitely sensitive microphone, more energy is expended moving the diaphragm than is used to vent the pressure wave out of the cell. Such a cell is described by Kreuzer, L. B., J. Appl. Phys., 42 (7), June 1971, p. 2934-2943.
The pressure-time response of the short (5 cm) cell described by Kreuzer operating in the windowless mode is such that only a very weak signal will be detected requiring very sensitive equipment. Immediately after the source beam is "turned on" to full intensity, analyte gas may absorb energy from the source light beam. Absorbed energy will be almost instantaneously transferred to translational energy, causing a temperature and pressure rise in the cell. The pressure rise in the cell will be sensed by the microphone diaphragm producing a momentary signal as the microphone diaphragm expands against ambient gas pressure. In response to the thermally perturbed environment in the cell, gas will flow out of the cell as a pressure wave at the speed of sound. The amplitude of the pressure wave will be such that thermal equilibrium will be established between gas in the cell and the environment according to the universal gas law. For the 5 cm long cell, approximately 0.074 millisecond is needed for the pressure wave originating at the center of the cell to exit either end of the cell. Assuming a 50% duty cycle modulation of the light source beam, the beam will transverse the cell during the "on-state" for 1.25 millisecond. Therefore, the pressure pulse will be detected for approximately 6% of the total "on-time" of the light source. Likewise, when the laser is "turned off" another pressure wave is generated by gases re-entering the cell. Because the pressure wave exists for only 6% of the total time the light source is on (or off), a relatively small signal will be observed from the microphone when operating a "short" cell in the windowless mode.